The present invention relates in general terms to methods of heat-treating steel, such an annealing, heating prior to quenching and tempering, in which it is important that the chemical composition of the surface of the metal should be altered and therefore that oxidation, de-carburization and carburization of its surface should be prevented. Such treatments generally take place in furnaces in the presence of a predetermined controlled atmosphere.
For treatments of this type, there have already been used atmospheres formed by gases which do not react to any appreciable degree with steel at the temperatures employed, which gases may be nitrogen or a mixture of nitrogen and hydrogen. In practice however, when nitrogen is used alone, it is difficult to avoid surface oxidation, while a mixture of nitrogen and hydrogen, although able to prevent such oxidation, does not generally allow surface de-carburization to be avoided. In other words, these known methods do not allow the desired results to be achieved.
Also known are methods of heat-treating steel of the aforementioned type in which the atmosphere used is formed by mixing a carrier gas (nitrogen alone or a mixture of nitrogen and hydrogen) with a hydrocarbon having the general formula C.sub.x H.sub.y. However, the type of atmosphere in which the carrier gas is formed by nitrogen alone usually leads to the formation of soot deposits at temperatures equal to or lower than 850.degree. C., while atmospheres in which the carrier gas is formed by a mixture of nitrogen and hydrogen give very erratic results because of the difficulty of accurately regulating the carburizing activity or carbon potential of the said atmosphere. Any error in the regulation of this carburizing activity results in the metal being treated either carburized or de-carburized to an excessive degree.
It is accordingly an object of the present invention to remedy or minimize the above-mentioned shortcomings of known methods.
Studies made of the de-carburizing activity of hydrogen have shown that this element, when dry, has virtually no de-carburizing activity below 900.degree. C. Wet hydrogen on the other hand, is a powerful de-carburizer. In other words, its de-carburizing activity is chiefly due to the presence of water.
When an atmosphere containing hydrogen is used in an industrial furnace, it is very difficult to prevent water from forming, as a result of the presence of oxygen and oxides in the furnace.
The result is that, when a mixture of nitrogen and hydrogen, even a very dry one, is fed into an industrial furnace, a mixture of nitrogen, hydrogen and water always forms in the furnace.
It is also known that hydrocarbons have the characteristic of reducing water in accordance with a reaction of the following type: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
that is to say by generating carbon monoxide and hydrogen.
These reactions take place at temperatures of the order of 650.degree. to 900.degree. C.
The problem which the invention is intended to solve, with, as mentioned above, the object of remedying or minimizing the shortcomings of known methods, is thus on the one hand to prevent the formation of water in the furnace, which inevitably results in decarburization, and on the other hand to avoid an excess of hydrocarbon which would inevitably lead to excessive surface carburization.